As highly-sensitive field sensors, fluxgate magnetometers (fluxgate sensors) are conventionally known. Such a fluxgate magnetometer is designed to sense, based on nonlinearity of a magnetization curve of a magnetic substance, the magnitude of a magnetic field to be measured; this magnetic substance is excited by an alternating magnetic field on which the magnetic field has been superimposed.
FIG. 10 illustrates the schematic configuration of a fluxgate magnetometer 10 having two orthogonal sensing axes and that of a signal-processing unit 100 operative to process detection signals output from the fluxgate magnetometer 10.
The fluxgate magnetometer 10 is provided with a ring core 11 made of a given magnet substance, a drive control winding 12 wound around the entire circumference of the core 11, and first and second sense windings 13 and 14 diametrically wound around the core such that they are orthogonal to each other and to the center axis of the core 11. The sensor construction of the fluxgate magnetometer 10 provides the two sensing axes.
Note that, in FIG. 10, illustrative representation of part of the second sense winding 14 is omitted to simply show the configuration of the fluxgate magnetometer 10. The sensing axis corresponding to the first sense winding 13 (top and bottom direction in FIG. 10) will be referred to as X axis, and that corresponding to the second sense winding 14 (right and left direction in FIG. 10) will be referred to as Y axis.
Excitation signals, such as excitation voltages, vd1 and vdd2 respectively having complementary waveforms and each having a previously set drive frequency fd are configured to be applied to the drive control winding 12 via resistors R1 and R2 connected to both ends thereof. To the respective first and second sense windings 13 and 14, bias voltage VBIAS is configured to be applied.
To the first sense winding 13, a resistor Rx and a capacitor Cx constituting a resonant circuit 15 together with the first sense winding 13 are connected. Similarly, to the second sense winding 14, a resistor Ry and a capacitor Cy constituting a resonant circuit 16 together with the second sense winding 14 are connected. The resistors Rx and Ry serve as limiting elements respectively to limit the Q factors of the resonant circuits 15 and 16.
The resonant circuits 15 and 16 are designed to resonate at a carrier frequency fc double of the drive frequency fd so that the first and second sense windings 13 and 14 are designed to detect signals (detection signals) vox and voy each having the carrier frequency fc.
In the fluxgate magnetometer 10 with the configuration set forth above, application of the excitation signals vd1 and vd2 to the drive control winding 12 allows an alternating current with the drive frequency fd to flow through the drive control winding 12. The flow of the alternating current through the drive control winding 12 generates an alternating magnetic field with the drive frequency fd in the core 11 along the circumferential direction thereof.
When there is no target magnetic field with an X-axis component and a Y-axis component, the magnetic fields in the two portions of the core 11 at which the first sense winding 13 is wound are cancelled to each other. Similarly, the magnetic fields in the two portions of the core 11 at which the second sense winding 14 is wound are cancelled to each other.
This allows the magnetic flux linkages across the first and second sense windings 13 and 14 to be substantially zero, so that the amplitudes of the detection signals vox and voy are substantially zero.
In contrast, when there is a target magnetic field with an X-axis component and a Y-axis component, the magnetic fields in the two portions of the core 11 at which the first sense winding 13 is wound are unbalanced. Similarly, the magnetic fields in the two portions of the core 11 at which the second sense winding 14 is wound are also unbalanced.
The unbalance of the magnetic field in the core 11 causes signal components to be induced in the first and second sense windings 13 and 14. The nonlinearity of the magnetization curve of the core 11 causes the induced signal components to have the carrier frequency fc double of the drive frequency fd.
The filtering functions of the resonant circuits 15 and 16 allow the signal components each with the carrier frequency fc to be sampled as the detection signals vOX and vOY. The amplitudes of the detection signals vox and voy represent the magnitudes of the X-axis and Y-axis components of the target magnetic field.
Note that the resonant circuits 15 and 16 practically cannot eliminate all components with frequencies except for the carrier frequency fc. As illustrated in FIG. 11, therefore, noise components each with a frequency substantially equal to the drive frequency fd (=fc/2) appear. The noise components become strongly apparent relatively when there is no target magnet field (see the waveform of the detection signal vox in the case where there is no target magnetic field Hx, such as Hx is zero, in FIG. 11). The noise components are normally superimposed on the detection signals independently of the magnitudes of the X and Y-axis components of the target magnetic field.
Note that the detection signals vox and voy obtained by the fluxgate magnetometer 10 can be regarded as signals obtained by performing amplitude modulation on a carrier wave with the carrier frequency fc based on the target magnet field as an original modulating signal (original baseband signal). For this reason, demodulation of the detection signals vox and voy allows a signal indicative of the target magnetic field to be sampled.
The detection signals vox and voy are normally subjected to digital processing. For example, in direction sensors for producing a direction signal indicative of a direction based on the detection signals vox and voy obtained by sensing the earth's magnetic field, direction obtaining operation and/or magnetizing correction, which are difficult to be carried out by analog circuits, have been performed by digital processing.
The signal-processing unit 100 operative to manipulate the detection signals output from the fluxgate sensor 10 is therefore provided with an X-axis processor 110 and a Y-axis processor 120 that demodulate the detection signals vOX and vOY to extract modulating signals (baseband signals) to convert the modulating signals into digital data.
Specifically, as illustrated in FIG. X, the X-axis processor 110, whose functions are substantially similar to the Y-axis processor 120, is operative to perform synchronous detection on the detection signal vox.
For example, the X-axis processor 110 turns on and off an analog switch 111 at a frequency equivalent to the carrier frequency fc and at proper timings; this analog switch 111 is provided in a signal path for the detection signal vox. As an example of the proper timings, the X-axis processor 110 turns on and off the analog switch 111 at the timings that allow only positive or negative half cycles of the detection signal vox to pass therethrough.
Next, the X-axis processor 110 smoothes the output signal of the analog switch 111 using a low-pass filter 113 consisting of a resistor and a capacitor, thereby obtaining a direct current (DC) voltage.
In addition, the X-axis processor 110 is operative to convert the DC voltage into digital data using an analog-to-digital (A/D) converter 115, thereby obtaining the amplitude AX of the detection signal vox. Like the X-axis processor 110, the Y-axis processor 120 is operative to perform the synchronous detection on the detection signal voy and the A/D conversion operation, thereby obtaining the amplitude AY of the detection signal voy.
Note that various types of methods of demodulating the detection signals vox and voy are conventionally well known except for the synchronous detection set forth above, some examples of which are disclosed in the following nonpatent literatures:
M. H. Acuna, “Fluxgate magnetometers for outer planets exploration” IEEE Trans. Magn., Vol. 10, pp. 519-523, 1974;
Makoto Kawakami, Hazama Takeshi, “Small Size Magnetic Field Sensor”, Technical Report of SUMITOMO TOKUSHU KINZOKU, Vol. 14, pp. 109-112, 2003;
X. Qianl, X. Lil, Y. P. Xu and j. Fanl, “Integrated driving and readout circuits for orthogonal fluxgate sensor” IEEE Trans. Magn., pp. 3715-3717, 2005; and
I. Sasada, “Symmetric response obtained with an orthogonal fluxgate operating in fundamental mode” IEEE Trans. Magn., vol. 38, pp. 3377-3379, 2002.
Any of the various types of methods use analog detection circuits in order to demodulate the detection signals vox and voy.
Specifically, the X and Y-axis processors 110 and 120 for performing demodulation on the detection signals vox and voy to extract original baseband signals and converting the extracted signals into digital data are designed to analog circuits.
This may make it difficult to integrate the analog circuits 110 and 120 together with the remaining elements of the signal-processing unit 100 in/on one chip, thereby preventing an apparatus designed to use such a fluxgate magnetometer from decreasing in size and manufacturing cost.
The analog X and Y-axis processors 110 and 120 consist of a large number of passive components that sensitively change in characteristics with time and with temperature. This may limit the accuracy of measurement of a target magnetic field and/or the reliability of an apparatus designed to use such a fluxgate magnetometer.